For example, U.S. Patent Application Publication No. 2003-3202051 corresponding to Japanese Patent Application Laid-open No. 2003-326712 discloses an ink jet head that discharges ink to form an image on recording paper. This ink jet head has a laminated structure formed by laminating thin metal plates. The bottom layer of the ink jet head is a nozzle plate having a large number of nozzles for discharging ink, which are formed through the nozzle plate by press working or laser processing. The ink jet head also has ink channels formed therein, each of which communicates with one of the nozzles.
However, the ink discharged from a nozzle of such an ink jet head may deviate directionally from the prescribed direction. This may be caused by positional shifting between plates that occurs when bonding and securing the plates, displacement of the nozzle axis during nozzle formation, and adhesion of foreign matter to the inner wall of the nozzle. The directional deviation leads to a significant reduction in quality of the image to be formed. Accordingly, if the directional deviation is greater than a predetermined or specified amount, the ink jet head is regarded as defective. There may be a case where the recording paper is positioned at a distance of 1 millimeter below the nozzle plate. In this case, the directional deviation is regarded as greater than the predetermined amount if ink droplets discharged from the nozzle land outside a circular region on the paper that has a radius of several tens of micrometers and a center aligned with the nozzle.
Several hundred to several thousand nozzles are formed in such an ink jet head. The head is rejected if the directional deviation at just a single nozzle is greater than the specified amount. Accordingly, as the number of nozzles increases, the head production yield falls.